Electrical filters are well known in the art. Filters are generally grouped as either lowpass, highpass, bandpass, or notch (also known as bandstop) filters. Lowpass filters suppress electrical signals above a particular desired cutoff frequency, passing only signals below, or lower than, the cutoff frequency. Highpass filters suppress electrical signals below a particular cutoff frequency, passing only signals above, or higher than, the cutoff frequency. Bandpass filters pass electrical signals between two cutoff frequencies. Notch, or bandstop, filters suppress electrical signals between first and second cutoff frequencies.
Implementation of the various types of electrical filters is also well known in the art. Depending upon the performance specifications required of a filter, electrical signal filtering can be performed using either passive components such as resistors, capacitors, and/or inductors, but may also include certain active components as well.
At relatively low frequencies, i.e., below 200 MHz, electrical filters are typically comprised of passive components and are usually so-called lumped elements, i.e., inductors are typically wire-wound devices and capacitors are typically parallel plate devices separated by either air or some other dielectric material. It's well known that at high frequencies, i.e., above 200 MHz, lumped elements do not behave very well, i.e. electrical characteristics are affected by many factors including the physical dimensions of the devices and their physical layout. At high frequencies, even a length of lead wire on a wire-wound inductor will itself have inductance that adds to the inductance of the coil windings and is an inductance which must be taken into account in the design and manufacturing of the device.
So called ceramic block filters have recently become popular in many applications because of their performance characteristics at high frequencies, their manufacturability, their reduced size (compared to lumped elements) and their inherent ruggedness. Ceramic block filters are well suited to perform either lowpass, highpass, bandpass, and bandstop functions at high frequencies. These devices are particularly well suited at high frequencies because they typically employ quarter wavelength sections of transmission line to achieve the functions of discrete or lumped components used at lower frequencies.
Ceramic bandpass filters are well known in the art and have been the subject of numerous patents in the United States. These devices are typically comprised of several quarter-wavelength sections that are configured to pass a relatively narrow band of signals and reject signals outside this band of frequencies. When implementing a bandpass filter in a monolithic block of material, (i.e., a single solid block of material) interstage coupling of passband signals improves the filters characteristic response by coupling more of the desired frequency signals from an input terminal to an output terminal while suppressing signals outside the passband.
In a bandstop or notch filter that suppresses signals between two frequencies, a bandstop filter that uses several cascaded stages can provide wider, more highly attenuating stop bands, than a filter using only one notch filter stage. In a multistage notch filter, interstage signal coupling of signals can permit undesired frequency signals to leak or couple from the filter input to the filter output. Depending upon the desired characteristics of a multistage notch filter, optimum performance can frequently be realized only when signal coupling between stages (interstage signal coupling) is minimized. Minimizing the interstage signal coupling between stages in a multi-stage notch filter improves the performance of the filter by having all of the signals to be suppressed, pass through the succeeding stages of the filter, each of which further attenuates undesired signals, further reducing their energy levels at the filter output. Stated alternatively, if a signal to be attenuated is allowed to couple from an input port of a filter to an output port of the filter, bypassing filter stages, signal attenuation will be reduced because of the filter stages that the signals bypass.
In monolithic ceramic block filters, a certain amount of coupling from the input port to the output port always exists by virtue of the fact that the filter is comprised of a single block of material from which some capacitance between an input terminal and an output terminal will always be realized. In the prior art, multistage ceramic notch filters used stages that were physically isolated from each other to achieve electrical isolation. Electrical isolation between stages in a multi-stage ceramic notch filter was typically accomplished by physically separating stages into several blocks, each block being electrically isolated by metal shielding provided by some type of sheet metal or physical distance separating the succeeding stages such that input signals could not readily couple to the filter output.
In the prior art wherein successive stages in a multistage notch filter were physically separated from each other, space was wasted separating the stages from each other but more importantly, filter manufacturing was more difficult and hence more costly. In applications where circuit board space is at a premium and where a multistage notch filter is called for, a multistage ceramic filter that is embodied within a single or monolithic block of material would be an improvement over the prior art. Accordingly, a monolithic ceramic block filter that has a notch or a bandstop response characteristic, that is implemented in a single block of material, and that improves isolation between filter stages without having to rely on physical spacing and/or shielding between stages would be an improvement over the prior art.